Nitride and oxynitride phosphors having a stable crystal structure have drawn attention in recant years as phosphors that are excellent in durability and that exhibit little drop in luminance accompanying risos in temperature.
Sialon, which is a solid solution of silicon nitride, is a representative instance of nitride and oxynitride phosphors. Like silicon nitride, sialon has two crystal systems, namely α-type and β-type. As is known, β-type sialon activated with a specific rare earth element exhibits useful fluorescence characteristics, and the use of β-type sialon as a phosphor in white LEDs is under ongoing study.
Patent Document 1 indicates that a phosphor having a main component in the form of p-type sialon containing Eu and represented by formula Si6-xAlxOxN8-x is excited over a broad wavelength region, from ultraviolet light to visible light, and emits, with high efficiency, green light having a dominant wavelength ranging from 520 nm or longer to 550 nm or shorter; as a result, the phosphor is excellent as a green phosphor. Patent Document 1 discloses as well various light-emitting elements that rely on this phosphor, singly or in combinations with other phosphors, and indicates that the phosphor can be used particularly appropriately in a white LED having an ultraviolet LED or a blue LED as a light source.
Patent Document 1 discloses a first process of generating β-type sialon containing Eu, and a second process of performing a thermal treatment, at respective optimal temperatures and times, in a nitrogen atmosphere, in vacuum, or in an inert atmosphere having a gas other than nitrogen as a main component.
The effect elicited by relying on these processes is as described above; in particular, it is stated that adopting the second process makes it possible to lower the nitrogen partial pressure in a heating treatment atmosphere, and to break down unstable nitride and oxynitride phases that have a high concentration of crystal defects.
Patent Document 2 discloses a green phosphor having a lowered concentration of oxygen in an oxynitride crystal having a β-type Si3N4 crystal structure, wherein superior green fluorescence is afforded in that the phosphor emits sharper light, of a narrower peak, than that of conventional sialon phosphors. In particular, Patent Document 2 (Tables 6 and 7) indicates that emission efficiency tends to increase when the Eu concentration is high and the oxygen concentration low in the phosphor (for instance, Examples 21 to 23).
Patent Document 1: WO 2008/062781
Patent Document 2: Japanese Patent Application Publication No. 2008-303331